The present invention relates to a system and method for controlling a direct injection spark ignition internal combustion engine.
Direct injection spark ignition (DISI) internal combustion engines may be operated in various modes depending upon the particular objectives to be attained at any particular time with emphasis on power output, fuel economy, and/or low emissions, for example. Operating modes may include a homogeneous mode in which the combustion chambers contain a substantially homogeneous mixture of air and fuel, or a stratified mode in which the combustion chambers contain stratified layers of different air/fuel mixtures. Stratified mode generally includes strata containing a stoichiometric air/fuel mixture nearer the spark plug with lower strata containing progressively leaner air/fuel mixtures.
Typically, there is a range of air/fuel ratios within which stable combustion can be achieved in the stratified mode, such as between 25:1 and 40:1, and a second range in which stable combustion can be achieved in the homogeneous mode, such as between 12:1 and 20:1. As such, there is typically a significant gap between the leanest air/fuel ratio of the homogeneous mode (20 in this example), and the richest air/fuel ratio of the stratified mode (25 in this example). This gap poses a number of problems in selecting.an appropriate operating mode and controlling the engine.
For example, best fuel economy is often associated with highest allowable manifold pressure which may dictate an air/fuel ratio which falls within the gap and is therefore not achievable in either mode of operation. As such, the engine controller operates the engine at a richer air/fuel ratio to maintain stable combustion with a resulting lower fuel economy.
The air/fuel ratio gap between operating modes also poses control challenges to avoid limit cycle behavior in which a small variation in requested torque causes cycling between stratified and homogeneous modes which have a large variation in associated air/fuel ratios.
An object of the present invention is to provide a system and method for controlling a DISI engine using a hybrid operating mode based on operation of the engine over a number of engine events, such as cylinder firings or cycles.
In carrying out the above object and other objects, advantages, and features of the present invention, a method for controlling a spark ignited direct injection internal combustion engine having a plurality of cylinders operable in at least a homogeneous operating mode with a homogeneous air/fuel mixture and an associated range of allowable homogeneous air/fuel ratios and a stratified mode with a stratified air/fuel mixture and an associated range of allowable stratified air/fuel ratios is provided. Typically, the homogeneous range and stratified range of allowable air/fuel ratios do not overlap and are widely separated. The method includes determining a desired value for an engine operating parameter based on current engine operating conditions wherein the desired value results in scheduling of an air/fuel ratio between the homogeneous range and stratified range of allowable air/fuel ratios. The method also includes operating a first portion of the cylinders in the homogenous operating mode, and operating a second portion of the cylinders in the stratified operating mode such that a combined air/fuel ratio associated with the first and second portions of the cylinders approaches the scheduled air/fuel ratio. In one embodiment, the engine operating parameter is engine torque. The operating mode of each cylinder is selected to provide a desired average engine torque over multiple engine events, such as cylinder firings.
The present invention provides a number of advantages over prior art control strategies. For example, the present invention provides an additional degree of freedom in torque control without additional sensor/actuator cost. The present invention provides an alternative strategy to multiple injections to effectively close the air/fuel ratio gap between stratified and homogeneous operating modes to increase engine efficiency.
The above advantages and other advantages, objects, and features of the present invention, will be readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.